1. Field of the Invention
The invention relates to a comminuting unit of a comminuting machine for comminuting material, in particular, to a knife block of a wood slicer, comprising a plurality of knife carriers arranged along a comminution path. The knife carriers are aligned with their longitudinal axes transversely to the comminution path and parallel to one another. Each knife carrier has an axis-parallel slicing knife at its leading longitudinal edge whose cutting edge extends at a predetermined angle and a predetermined projecting length into the comminution path, wherein the slicing knife of a knife carrier together with the rearward longitudinal edge of the preceding knife carrier defines an axial passage slot in the comminution path.
2. Description of the Related Art
Comminuting units of the aforementioned kind are used, for example, in knife ring slicing devices with a stationary or a rotating knife ring and serve, inter alia, for comminuting wood which may be provided in the form of tree trunks or also in the form of chopped strands. The strands which are produced in this way serve as a starting material for wood-based structural boards, such as, for example oriented strand board (OSB) or particle board.
In the manufacture of particle board as well as oriented strand board (OSB), the strands are wetted with an adhesive, distributed on a support, and pressed to sheet-shaped structural parts by applying heat and pressure. The deciding factor for the strength and economic efficiency of such products is the quality of the strands. The quality is determined primarily by the quality of the cut surfaces. When the cut surfaces are smooth, the surface area of the strands to be wetted is reduced so that less bonding agent (adhesive) is required for bonding the material. Moreover, smooth cut surfaces provide planar contact surfaces during adhesive bonding which leads to an improved connection of the strands with one another and thus results in a higher strength of the finished structural part.
A further factor in regard to the strand quality, which is important primarily in regard to oriented strand board, is the requirement of a uniform geometry. With uniform strand dimensions, it is possible to distribute the strands with almost fiber-parallel alignment so that very high strengths can be obtained in the bearing or loading direction. While the thickness of the strands can be easily produced by means of the projecting length of the cutting edge and the strand length by means of scoring knives, the manufacture of a constant strand width requires a more complex approach.
When removing a strand from the residual wood to be chipped by means of a slicing knife, the cutting edge of the slicing knife penetrates by the projecting length of the cutting edge into the material and slices off a strand of the material like a plane. When doing so, an advancing wedge-shaped tear or crack between the residual wood and the strand being sliced off will result immediately in front of the cutting edge of the slicing knife. A problem in this connection is that the wedge-shaped tear prevents the formation of a smooth cut surface and, according to the shape and the course of the longitudinal fibers of the material, results in roughness of the cut surface. Moreover, an increased amount of bonding agent is required for producing the particle board or oriented strand board. Also, the strength of the produced boards is reduced.
In order to counteract the formation of a wedge-shaped tear and the resulting negative effects, attempts have been undertaken to limit the area of the wedge-shaped tear to the immediate cutting edge area. For example, German patent 32 05 759 C1 describes a slicing device comprising a knife block in which knife carriers with the corresponding slicing knives are arranged in a ring shape. Two sequentially positioned knife carriers form by means of the back of the first knife carrier and the lower longitudinal edge of the leading knife carrier a passage slot for the strand. The rearward edge of the leading knife carrier is also referred to as a pressure lip which exerts a radial pressure acting on the body to be sliced and, in this way, reduces the degree to which formation of a wedge-shaped tear occurs. Still, in this way a tear formation cannot be prevented so that an improvement of the strand surface relative to slicing without pressure lip is achieved; however, the strands which are produced in this way have still a certain roughness at their cut surface which leads to the afore described disadvantages.
A further disadvantage of the passage slots known from the prior art is caused by their constructive configuration. In order to prevent clogging of the passage slot, the passage slot widens extremely behind the pressure lip so that the effect of the passage slot, i.e., providing a passage of a predetermined size, is present only within the plane of the slot. This has the result that upon comminution of small-size material such as, for example, chopped strands, parts of a greater dimension than that of the slot width will be squeezed through the slot which can lead to splintering of the material. On the other hand, it frequently happens that partial pieces that are too large will pass through the slot as a result of the elasticity of the material and the strands manufactured in this way fluctuate greatly with regard to size and shape.